1. Field of the Invention
This invention relates generally to test instrumentation for television systems, and more particularly to instrumentation for measuring hum and other forms of AM distortion in television signals.
2. Description of the Prior Art
Hum and other forms of AM distortion are introduced into television signals from a variety of sources, such as power supplies, amplifiers and other sources within cable television distribution systems, as well as AC power sources near components of television systems. Composite hum can be measured with existing meters. One technique, used in the cable television industry, involves transmitting an unmodulated test carrier on an unused cable channel and then measuring amplitude variations in the signal on that channel at some remote point in the system. A shortcoming of this technique is that many cable service providers use all available channels for commercial programming and do not welcome interruption of service on any channel for test purposes.
An instrument designed for hum testing on channels in use is described in U.S. Pat. No. 4,760,448. This instrument samples the high and low peak amplitudes of horizontal sync pulses in a composite television signal and determines the percentage of hum in the RF input signal on the basis of the difference between the maximum and minimum sampled levels. The disclosed sampling technique requires circuitry capable of separating the horizontal and vertical sync pulses from the detected composite video signal, generating a train of sampling pulses in sync with the horizontal sync pulses, and then interrupting the pulse train during the vertical blanking interval so as to prevent sampling of the detected video signal during that interval. Sampling must be inhibited during the vertical blanking interval because control signals are transmitted then at varying amplitude levels which would otherwise cause the hum measurements to be inaccurate. While such an instrument with its selective sampling circuitry may be adequate in certain applications, it does not operate properly on channels on which the signal is scrambled by means of sync suppression, which is the most common scrambling method used in the cable television industry.
Thus, there remains a need for a versatile method and apparatus for measuring AM distortion on active channels in an RF distribution system. A further need exists for enhanced system troubleshooting through tracking of different distortion components.